Davies’ disc engine uses a disc fixed to a shaft that swings in a conical circuit about a central spherical bearing. The shaft end connects to a rotating crank. Hiscox describes conical cylinder heads and a spherical outer shell over which the disc rim moves.
Davies' Disc Engine Interactive Calculator
Explore the disc’s nutation inside conical heads and a spherical outer case. Adjust disc diameter, shaft cone angle and crank speed. The illustrated shaft length from center pivot to crankpin is 0.9 times disc diameter.
Equation Used
- Rigid thin circular disc centered on a spherical pivot.
- Fixed shaft cone half-angle; one nutation cycle per crank revolution.
- Illustrated pivot-to-crankpin shaft length is 0.9D.
- No chamber-displacement, indicated-power or torque estimate.
The shaft nutates about a fixed central spherical pivot and connects to a rotating crank. The disc stays perpendicular to that shaft. The case is drawn transparent for visibility.
A disc that nutates around a fixed center
The animation turns the historical arrangement upright for a clear oblique view. The center of the blue disc remains fixed while its normal shaft traces a cone. The shaft tip follows the gold crankpin orbit. The disc’s rim remains on a sphere centered at the pivot.
The wireframe heads are cones whose apex meets the central bearing. They bound the allowed disc inclination. The outer curved case is transparent. Hiscox states that steam enters alternately on either side of the piston. Steam distribution and valve timing are outside this geometric illustration.
Explore the nutating geometry
Disc diameter and cone angle control the axial extent of the moving rim and the projected elliptical outline. The shaft-to-crank connection is shown with an explicit pivot-to-pin length of 0.9D. This proportion is illustrative rather than a dimension taken from the historical engine.
The speed control changes the frequency of the nutation. The diagram explains the linkage; it does not prescribe a working pressure, operating-speed limit or manufacturing clearance.
Geometric results
With disc diameter D and cone half-angle β, opposite tilted rim positions span an axial range D sin β. Looking along the fixed crank axis, the thin disc projects as an ellipse with major diameter D and minor diameter D cos β.
A shaft of length L from the center pivot to the crankpin has orbit radius L sin β and axial reach L cos β. Here L=0.9D, so the displayed orbit diameter is 1.8D sin β. Nutation frequency is crank rpm divided by 60.
These dimensions do not establish swept chamber volume. Multiplying disc face area by rim-height range does not model the changing wedge-shaped working chambers.
A 200 mm disc at 20 degrees
The axial rim-height range is 68.40 mm. The projected ellipse has minor diameter 187.94 mm. The illustrated 180 mm pivot-to-pin shaft gives a crank orbit diameter of 123.13 mm and axial reach of 169.14 mm. At 300 rpm there are 5 nutation cycles per second.
Reducing speed to zero stops the animation without changing any geometric result.
Geometry is not an engine performance model
A power calculation requires the actual working-chamber volumes, pressure cycle and steam distribution. The disc’s rim-height range alone does not determine changing chamber volume, shaft power or torque.
Real disc thickness, central bearing size, rim sealing and the crankpin joint require detailed design. The source drawing is a mechanism description, not a dimensioned manufacturing specification. No service life or universal clearance is claimed.
Disc-engine questions
Does the disc center travel?
No. The central spherical bearing holds its center while allowing its orientation to change.
Why is the case curved?
Every rim point is one disc radius from the fixed center, so the moving rim lies on a sphere.
What does the angle slider measure?
The angle between the moving disc shaft and the fixed crank axis. It is also the inclination of the disc plane from a plane perpendicular to that fixed axis.
Can this determine steam power?
No. It computes geometric dimensions and cycle frequency only.
Original mechanism reference
Gardner D. Hiscox, Mechanical Movements, Powers, Devices and Appliances (1901), printed page 96, movement 352, disc, oscillating shaft, central ball bearing, disc crank, conical heads and spherical shell.
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